Why do people buy a container made for one size canopy and promptly put in a larger/smaller canopy? Maybe I'm getting cranky in my old age, but that has always puzzled me.

If you limit your downsizing according to Brian Germain's .1/100 jump formula, you need to spend 100-200 jumps on each size.

If you're making 150-200 jumps a year, limiting container usage to the designed size and one smaller gets you 2 years of life out of that expensive new rig.

Stuffing one bigger in there gives you 50% more useful life. Of course, this disregards that some rig designs do a good enough job (tight closing loop, but the side flaps close at a less attractive angle) with a couple sizes smaller.

Note that nearly all skydivers find a wing loading they'll be happy with indefinately (or at least until technology changes). When you hold off buying custom gear until after you've reached that point, you can get a _lot_ of life out of it even if you grow a beer belly or shrink one. I've had my first custom rig for six years and spent about eight years with the same sized main+reserve containers.